The present invention relates to hydraulic machines and in particular to improvements in runner blades for Francis-type machines.
Conventional Francis-type runners comprise a crown with a concentric band or ring and an array of blades extending between and interconnecting the crown with the ring. The runner is rotated about the axis of the crown by the action of water passing through a plurality of flow paths formed between each pair of adjacent blades in the array of blades. Each of the blades has a leading flow dividing edge which defines the line of demarcation between a suction surface leading in the direction of rotation and a trailing pressure surface trailing in the direction of rotation. This edge, at the inlet of the blades, is normally oriented to slope from the band toward the crown to advance in the direction of rotation of the runner. Water enters the periphery of the runner and passes over the blades and leaves the runner axially at the trailing outlet edges of the blades.
More recently, WO 98/05863 published Feb. 12, 1998 to Bilidal et al discloses a runner for a Francis-type hydraulic turbine where each blade has an inlet edge and an outlet edge. The blade is characterized, in the direction of turbine rotation, by the junction point of each blade inlet edge at the band being located forwardly of the inlet edge attachment point for that blade at the crown, and the junction point of each blade outlet edge at the crown being located forwardly of the outlet edge attachment point for that blade at the band. Because of the twisting aspect between the inlet edge and the outlet edge of the blade, this blade is commonly referred to as the “X-blade”.
The blades of the aforementioned Francis-type runners may be subject to failure adjacent the junctions of the trailing outlet edge of the blade with the crown and band due to fatigue and/or high stress concentrations adjacent the junctions. In the past, to reduce stress concentrations in the runner blade adjacent these junctions, the blade had a cutout or notch located in the trailing edge of the blade adjacent each of the band and crown. However, this cutout may adversely affect the hydraulic flow of water past the blade which is not acceptable in some applications.
There is a need for an improvement in a runner blade that does not significantly alter blade contour, reduces stress concentrations adjacent the junctions of the blade outlet edge to the crown and the blade outlet to the band, and does not adversely affect the hydraulic flow of water past the blade.